The present invention and its technical background will be described herein in conjunction with high power X-ray tubes for use with high quality CT scanners and the like. It is to be appreciated, however, that the invention will also find application in conjunction with conventional X-ray diagnostic systems and other penetrating X-radiation systems for medical and non-medical examinations.
Typically, a high power X-ray tube includes an evacuated envelope or housing which holds a cathode filament through which a heating current or filament current is passed in order to serve as an electron emitter for thermionic emission of electrons. A high electrical potential, typically in the order of 100 to 200 kV, is applied between the cathode and the anode which are also located within the evacuated envelope. This potential causes a tube current or beam of electrons to flow from the cathode to the anode through the evacuated region in the interior of the evacuated envelope. The electron beam impinges on a small area of a focal spot of the anode with sufficient energy to generate X-rays. The X-rays may then be transmitted through an object to be observed such as a patient. While a portion of the X-rays will be absorbed within the object, the transmitted X-rays may be detected by an X-ray detector arranged at an opposite side of the object.
In order to increase the resolution of the CT scanner, it may be desirable to modulate a position of the focal spot between two or more positions, thereby creating two locally distinct point sources of X-radiation. High quality CT scanners may use a movement of the focal spot to double the resolution of the imaging system.
In a conventional X-ray tube design, a cathode is provided for emitting an electron beam towards a rotating disk-shaped anode such that a focal spot is generated on a slanted X-ray emitting surface of the anode. The generated X-rays are emitted in a direction substantially perpendicular to a direction of the impinging electron beam.
In such X-ray tube, it may be advantageous to provide a focal spot which can be moved in a direction of the anode's rotating axis in order to be able to generate two distinct focal spots. This direction typically coincides with a direction of the impinging electron beam and is usually referred to as y-direction. A direction perpendicular to the y-direction, i.e. the typical direction of the emitted X-rays from the anode towards the X-ray window of the X-ray tube and then towards the patient is usually referred to as z-direction. A direction perpendicular to both, the y-direction and the z-direction, i.e. a direction tangential to the rotating anode disk, is usually referred to as x-direction.
In such typical X-ray tube design, the desired movement of the focal spot and of the emitted X-ray beam in y-direction may be obtained by a movement of the electron beam in the z-direction, i.e. in a direction towards the detector.
Conventionally, two different methods have been employed to control and move the position and/or width of the focal spot of an X-ray tube.
One method of focal spot control employs electrostatic grids or biasing electrodes associated to a single electron emitting filament of the cathode. Voltages on the two electrostatic grids may be varied to change the location as well as the width of an electron beam impinging on the focal track of the rotating anode. However, such electrostatic grids for controlling both, the position and the width of a focal spot, may require a special complex and expensive grid design.
Another method of focal spot control may employ a magnetic yoke in order to create a magnetic field that affects a path of an electron beam emitted from the anode. However, the provision of magnetic yokes within a housing of an X-ray tube may require a special expensive design of the whole X-ray tube. For example, the magnetic yoke tube requires two additional connections to be passed through the X-ray tube housing, making it incompatible with many CT systems. In addition, the magnetic fields employed to deflect and focus the electron beam may not be moved in a square wave fashion between the two focal spot positions, thereby potentially creating a gap in the collected X-ray detection data.